Today's communications and remote sensing spacecraft include large flexible appendages such as solar arrays and antennas that vibrate when excited by thruster or instrument disturbance torques. These vibrations may result in out-of-tolerance antenna or instrument pointing errors.
Maintaining precision payload pointing with large flexible structures poses a significant challenge for spacecraft attitude control systems, since the motion of the appendages can adversely impact payload pointing performance. The increased size of the flexible appendages results in lower flexible mode frequencies that impact the bandwidth and stability robustness of attitude control systems. Low-bandwidth controllers that are designed to avoid interaction between the attitude control system and the spacecraft flexible dynamics may have insufficient performance to meet payload pointing requirements. In addition, low-bandwidth control schemes may not meet pointing requirements in the presence of disturbances that excite flexible structure vibrations. These disturbances include those induced by thruster firing during stationkeeping and momentum unloading maneuvers as well the payload scanning mirror on a remote sensing instrument. Advanced control methods are needed to increase the damping of flexible structural modes for payload pointing requirements that must be met in the presence of such disturbances.
Certain flexible spacecraft structure control systems rely on accurate a-priori models of the flexible spacecraft dynamics and involve ground-based controller design that can be time-consuming and expensive. Robustness is generally an issue due to the need to develop highly accurate models of all system elements between the applied control input and the measured system output. Designing robust control systems, which provide the required attenuation of flexible structure motion in the presence of model uncertainties, is thus a challenging and time consuming process. In addition, these systems require extensive redesign whenever the spacecraft dynamic models change.